1. Field of the Invention
The invention relates to an electrode connection between the sections of carbon or graphite electrodes in whose faces are sunk, for the accommodation of double-conical threaded nipples, threaded sockets of truncated-cone shape and rotation symmetrical recesses emanating from the socket base.
2. Description of the Prior Art
Graphite and carbon electrodes, hereinafter called graphite electrodes used in arc furnaces, particularly for the production of electric steel, consist of at least two, and usually a string of three or more cylindrical sections interconnected mechanically and electrically. The electrode is gradually consumed during the operation of the furnace, and as replacement of the losses caused primarily in the lower part of the electrode string, such as by evaporation of the carbon in the arc, erosion and oxidation, a new graphite section is periodically attached to the graphite section at the upper end of the electrode string. The various sections of the electrode string are screwed together, especially by means of double-conical threaded nipples screwed into double-conical recesses of truncated-cone shape--hereinafter called sockets--sunk into the faces of the electrode sections.
During the operation of the arc furnace, the electrode string is subjected to major stresses, e.g. due to radial temperature gradients, bending stresses due to scrap cave-ins and tensile stresses due to the electrode string's own weight. Since the nipple and electrode sections usually consist of two different graphite grades having different mechanical, thermal and electrical properties, the thermally induced stresses are particularly great in the connection zone, and under these conditions it is not possible to completely preclude fractures of the connection, causing graphite losses and especially furnace operation troubles. To limit the thermally induced stresses, it has been proposed to use nipples whose coefficient of thermal expansion is smaller in the radial direction than the coefficient of expansion of the adjacent electrode section measured in the same direction (German DE-PS No. 1,054 193 corresponding to British Patent Specification No. 889,252). Due to the anisotropic nature of the graphite part, a reduction in the axial tensile strength of the nipple and, above all, an increase in the generation of joulean heat in the nipple must be tolerated. Therefore, the disadvantages of such material changes usually counterbalance the advantages so that changes in the geometry of the nipple and the socket are preferred to relieve the nipple connections. The greatest stresses occur in the area of the nipple equator and especially in the transition zone between the socket base and the first thread turns of the socket in engagement with the nipple thread. In these areas, the accumulated stresses are often greater than the breaking strength of the graphite, and crack formation occurs and, in the extreme case, also breakage of the nipple, socket or both parts. Various solutions have been suggested to limit the build-up of stresses, especially in the transition zone from the socket base to the screw thread. For example, a connection is known from German DE-PS No. 25 55 688 in which the depth of the socket thread decreases steadily towards the socket base as does the depth of the nipple thread towards the nipple equator, thereby reducing stress peaks in the respective first thread turns. It is known from another suggestion to remove, by undercutting, the thread turns of the socket not in engagement with the nipple thread, thereby increasing the notch angle (German DE-AS No. 22 34 411 corresponding to British Patent Specification No. 1 388 746). The thread cross section is expediently shortened in the undercut zone, and the thread flanks facing the socket opening are cut at an angle no greater than a right angle due to the shortening (German DE-AS No. 26 57 952). The essential purpose of these measures is to reduce the stress on the socket in this critical area. According to German DE-OS No. 31 31 588, a relief of the thread turns from compressive stresses in the transition zone of the socket base and a reduction of the very great current density in this area is supposed to be achieved, finally, in that the resistance moment of the nipple is lessened by rotation-symmetrical recesses emanating from its face such that the nipple yields elastically when under radial bending stress. This recess extends at least over the height of the first three engaged thread turns, and their contour is designed so that the tangent with the nipple axis includes an angle greater than 0.degree. at any point. In another group of suggested solutions, socket and nipple were to be relieved particularly of tensile stresses by means of slots, grooves or holes. Slots extend essentially parallel to the electrode axis transversely through the socket wall or nipple (U.S. Pat. No. 2,527,294) or axis-parallel holes are machined into the socket wall (Austrian Pat. No. 259 888 corresponding to British Patent Specification No. 985,878).
Relieving local stress peaks in the joint is generally not sufficient to completely preclude breakage of the connection. Therefore, others have attempted to transfer active stresses into those parts of the electrode string which are stressed in compression. U.S. Pat. No. 3,646,240 discloses an electrode connection whose socket bases are recessed in vault-fashion. In the extreme case the vaulted recess is a hemisphere with the socket area as the cross section. Preferred are hemispheres with larger diameters than the socket. In this embodiment, the forces acting upon the connection are to be transferred into the electrode sections without the build-up of destructive stress concentrations. Another embodiment, (U.S. Pat. No. 4,161,619) provides for a flatter recess below the socket base, the last flank of the socket thread transitioning with uniform curvature into the plane limiting surface of the recess. The curvature radius is to be 12 to 18 mm, approximately.